Electrically actuated explosive device



Dec. 13, 1966 w. DAHL 3,2

ELECTRICALLY ACTUATED EXPLOSIVE DEVICE Filed Sept. 10, 1963 6 Sheets-$heet 1 Fig. 1.

INVENTOR.

WALTER L. DAH L Dec. 13, 1966 w. L. DAHL 3,291,046

ELECTRICALLY ACTUATED EXPLOSIVE DEVICE Filed Sept. 10. 1963 6 Sheets-Shet 2 I l I l I bit; L k;

INVENTOR.

WALTER L. DAHL Dec. 13, 1966 w. L. DAHL 3,291,046

ELECTRICALLY ACTUATED EXPLOSIVE DEVICE Filed Sept. 10, 1963 s Sheets-Sheet s INVENTOR.

WA LTER L. DAHL Dec. 13, 1966 w. DAHL 3,291,046

ELECTRIGALLY ACTUATED EXPLOSIVE DEVICE Filed Sept. 10, 1963 6 Sheets-Sheet 4 INVENTOR. WALTER L. DAHL Dec. 13, 1966 w, DAHL ELEGTRICALLY ACTUATED EXPLOSIVE DEVICE 6 Sheets-Sheet 5 Filed Sept. 10, 1963 INVENTOR. WALTER L. DAHL Dec. 13, 1966 w DAHL ELECTRICALLY ACTUATED EXPLOSIVE DEVICE 6 Sheets-Sheet 6 Filed Sept. 10, 1963 INVENTOR.

WA LTER L. DAH L United States Patent 3,291,046 ELECTRICALLY ACTUATED EXPLOSIVE DEVICE Walter L. Dahl, Woodbnry, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del., a corporation of Delaware Filed Sept. 10, 1963, Ser. No. 307,820 9 Claims. (Cl. 10228) This invention relates to electric initiators and, more particularly, to an electric ignition assembly which makes possible the production of electrical explosive initiators having high standards of performance and safety.

With the development of stringentrequirements for performance of explosive devices in both military and commercial applications, a need arose for an electric ignition assembly which is reliable in operation and easily adapted to mechanized production techniques, even in exceptionally small sizes. The ignition assembly should be highly resistant to accidental actuation by static discharges, stray current, and radio-frequency radiations. Although the firing characteristics of an ignition assembly of a particular design should be reliable and reproducible, changes in the firing characteristics should be possible without extensive redesign. The ignition assembly further should be capable of withstanding rough treatment and should demonstrate negligible deterioration in storage. A current military specification, for example, reqiures a small electric initiator, approximately A inch long and less than inch in outer diameter. The firing energy of such an initiator is required to be about 5,000 to 10,000 ergs at 40 to 100 volts, and the initiator should actuate a pellet of tetryl, pressed at at least 5,000 p.s.i., over an air gap of at least 3 to 10 mils. The initiator must be adaptable to mass production and should be resistant to premature firing by static discharges, stray current, and radio-frequency energy.

Prior to this invention, no initiator has been developed which has been able to meet such requirements on a mass production basis, primarily since no satisfactory ignition assembly has been available. Electric initiators of the conventional type have an ignition assembly comprising a pair of parallel electric conductors extending into the shell from an external firing circuit, a plug of a rubbery composition holding the conductors in place and sealing the initiator, a bridgewire connecting the ends of the conductors within the shell, and a heat sensitive ignition compostion in contact wth the bridgewire. In addition to the well known susceptibility of this assembly to accidental actuation by static discharges, stray current and radio-frequency radiation, such an assembly is not well adapted to miniaturization owing to the vulnerability to breakage of the extremely fine wires which would be required in the course of fabrication of the initiator and the sensitivity of such assemblies to double leg to shell electrostatic discharges. Graphite film bridges which have been proposed for use with twisted, insulated conductors molded in a plastic block are subject to deterioration in storage and are not able to withstand rough handling.

Another form of ignition assembly heretofore proposed involves the use of a central electrode, a conductive mix containing dispersed particles of a conductive material surrounding the electrode, and a metallic initiator shell which served as the other electrode. The conductive mix provides a plurality of conductive paths such that sufiicient energy to initiate the ignition charge is generated upon the passage of an extremely small electric current. Initiators having an ignition assembly of this type naturally are prone to accidental actuation by stray or static charges and further are prone to serious deterioration over a period of time. Further, such initiators are subject to 3,291,046 Patented Dec. 13, 1966 variations in initiation energy due to segregation and other sources of inhomogeneity inherent in mixtures of their heat sensitive and electrically conductive elements.

In accordance with this invention, there is now provided a novel electric ignition assembly which avoids the disadvantages of prior ignition assemblies and meets even more stringent requirement of reliability, Thi igniton assembly makes possble the production of explosive initiators and devices of high standards of performance and safety even in units of exceptionally small size, i.e., as small as 7 inch in length and as small as 6 inch in di ameter. Of equal importance, the initiator construction of this invention is highly suitable for mechanized production. This ignition assembly comprises:

(A) an essentally cylindrical insulating plug,

(B) a single electrical conductor extending axially through said plug,

(C) a sleeve of electrically conductive material in intimate peripheral swaged engagement with said insulating plug, and

(D) a heat-sensitive ignition composition contiguous to one end of said electrical conductor.

In the ignition assembly, the central conductor serves as one contact in the firing circuit and the conductive sleeve is the ground contact. Except in a flush type are gap ignition assembly, the central conductor will have at least one exposed or bared portion extending axially from the insulating plug, and the ignition charge will be contiguous to said exposed portion.

In an embodiment of this invention preferred for reliability, at least one electrically conductive element of high resistance is affixed at the end of the aforesaid exposed portion of the electrical conductor, said resistance element being in intimate contact with the ignition composition and having a portion of its length swaged to said sleeve to provide a homogeneous bond. Preferably, the resistance element is swaged between the plug and conductive sleeve.

In a preferred embodiment of this invention, the insulating plug is composed of two segments, viz., a cylinder of insulating material which peripherally engages the central conductor and a reinforcing sheath or capsule of rigid material such as metal or plastic in snug peripheral engagement with said cylinder. The reinforcing sheath or capsule strengthens the plug and aids in maintaining close electrical contact between the resistance element and the conductive sleeve or between the conductive sleeve and a ground wire should one be employed. The term insulating plug as used herein is intended to include a plug composed of such an insulating cylinder and reinforcing sheath or capsule.

In an embodiment of this invention particularly preferred for storage and handling, the central electrical conductor is formed in at least two segments, the part of the conductor adapted to extend into the shell and to which the resistance element is affixed being stationarily embedded within the insulating plug and the segment which extends to the external firing circuit being releasably mateable into or onto the stationary element.

The composite ignition-plug assembly of this invention is prepared by (1) providing an electrically conductive element which may be unitary or of two or more mateable sections, (2) surrounding at least part of the stationary element of the electrical conductor with a relatively rigid, essentially cylindrical insulating plug, the plug being molded or shrunk by heating about the conductor or provided with a preformed bore, (3) preferably, affixing at least one bridgewire to an extremity of the electrical conductor extending from the plug and adapted to be inserted into the initiator shell, (4) inserting the assembly so formed into a tubular sleeve of an electrically conductive material, which can in some embodiments be the initiator shell itself, the axis of the conductive sleeve and of the conductor-plug assembly being coincident and the bridgewire (if present) passing between the plug and the sleeve and (5) uniformly swaging the sleeve about the conductorplug assembly, the sleeve thereby being in close peripheral engagement to said plug and the bridgewire being firmly retained between the plug and the sleeve by an essentially homogeneous bond. As mentioned above, a supplementary reinforcing sheath of a rigid material may be positioned about the insulating material before the assembly is inserted into the conductive sleeve. It is preferred for ease of inserting the ignition assembly into the initiator shell that the conductive sleeve or a cylindrical retaining sleeve in peripheral engagement therewith be longer than the insulating plug and adapted to extend into the bore of the initiator shell. After assembly, the annulus between the conducting or retaining sleeve and the projection of the central conductor is filled with the ignition charge either in a separate loading :step or by inserting the ignition charge directly into theinitiator shell which has been previously loaded with base charge, priming charge and ignition composition. In instances in which the initiator shell is of metal and is adapted to serve as the ground contact in the firing circuit, the shell is loaded with the required explosive train including the ignition composition; the insulating plug carrying the central electrode to which are affixed the bridgewire(s) inserted with the bridgewire(s) drawn back so they are retained between the plug and the shell; and the shell, i.e., the conductive sleeve, swaged or sized to reduce the diameter of the shell about the plug assembly.

The ignition assembly of this invention can be employed in conjunction with any type of electric initiators such as detonators, i.e., blasting caps; squibs; military detonators; initiators for detonating fuse, low-energy connecting cord, or safety fuse; detonation-ignition cords such as Pyrocore; electric small arms and power cartridges; primers; and the like.

The ignition assembly of this invention is particularly advantageous for use in an explosively releasable fastener which is self-sealing before, during, and after actuation. A particularly preferred explosively-releasable fastener comprises an elongated fastener housing, i.e., shaft, adapted to engage with associated elements, said shaft having an axial bore closed at one end and containing in sequence from said end:

(a) a base charge of a high-velocity detonating explosive,

(b) a priming charge in propagating relationship to said base charge,

(c) an ignition assembly of this invention in propagating relationship to said priming charge, the conductive sleeve of said assembly being in contact with means for grounding the assembly,

, (d) a seal of a pressure-compactable material closing the bore in the shaft,

(e) rigid closing means reducing the bore in the shaft, and

(f) means extending through said rigid means and firmly engaged by said seal for connecting said electrical conductor of the ignition assembly to an external firing circuit. 1

The fastener can be released at a predetermined location without destroying or undermining the strength of adjacent structures and without producing shrapnel or in any way leaking in the presence of pressure from the severed end before or after actuation. This fastener does not require a separate closed breech system or any enlarged members which would materially add to the cost, size, and weight of the fastener. The fastener can be made of mild steel with a loading of base charge of only from 20 to about 100 milligrams of detonating explosive and a loading of priming charge of from to about 50 milligrams of primary explosive. The diameter of the bore of the fastener housing, i.e., shaft, is about A to /2 of the diameter of the housing at the section of the housing to be severed, i.e., the section of the housing contiguous to the base charge. The length of the'bore in a steel fastener containing such minute loadings may be only about inch to 1 inch and the diameter of the bore is about inch to about /3 inch.

The term fastener is intended to include bolts, pins, fasteners, electrical connectors, bonds, studs, screws, turnbuckles, shackes and the like.

The present invention will be better understood from the following description when considered in connection with the accompanying drawings.

Referring to the drawings:

FIGURE 1 is a sectional view on an enlarged scale of the ignition assembly of this invention shown in a detonator;

FIGURE 2 is a sectional view of a second embodiment of the ignition assembly of this invention shown in a detonator; '7 7 FIGURE 3 is a view of yet another embodiment of an initiator containing an ignition assembly in accordance with this invention;

FIGURE 4 is a sectional view of an initiator containing the ignition assembly of this invention modified for ease of assembly;

FIGURE 5 is a sectional view of an initiator containing a spark gap ignition assembly of this invention;

FIGURE 6 is a view of a fuse lighter which employs the ignition assembly of this invention;

FIGURES 7 and 8 are sectional views of connecting means for multiple section electric conductors;

FIGURE 9 is a cross-sectional view of a novel selfsealing explosively releasable fastener which employs the ignition assembly of this invention; and

FIGURE 10 is a cross-sectional view of the novel selfsealing explosively releasable fastener adapted for ease in loading.

In FIGURE 1, 1 designates the cylindrical shell which may be of metal or a hard plastic, the shell being open at one end and closed at the other end by bottom wall 1A. Within the shell are base charge 3 and priming charge 4. The ignition assembly comprises a central conductor 5 which is surrounded by an insulating plug composed of insulation 6, which is part of the conductor insulation, and reinforcing sheath 6A swaged on said insulation. Bared portion 5A of the conductor extends from the plug into the ignition zone of the initiator. Con ductive sleeve 7 peripherally engages reinforcing sheath 6A and retains ground wire 5B therebetween. Ground wire 53 preferably is a woven conductor which is unravelled and the separate filaments then fanned out over the surface of sheath 6A before conductive sleeve 7 is swaged into place. Optional retaining sleeve 8 which is, in this embodiment, of conductive metal surrounds the assembly and provides an annulus into which the ignition charge is loaded. Bridgewires 9 are affixed to the end of exposed portion 5A of the central conductor, e.g., by solder, welding, or staking. The free ends of .the bridgewire are swaged between conductive sleeve 7 and retaining sleeve 8. Ignition charge 10 surrounds the conductor 5A and the bridgewires. This construction has been used to prepare dual bridge detonators inch in diameter over the shell body by swaging sleeve 7 onto an insulated central conductor 5 and thereafter soldering the bridgewire 9 to extension 5A and swaging its free ends directly between sleeve 7 and initiator shell 1.

In FIGURE 2, the elements are essentially the same as the similarly numbered elements in FIGURE 1; however, in this embodiment, the conductor 5 is a contoured element and plug 6 is a mass of cured butyl rubber, Teflon, or irradiated polyethylene tubing. The conductor comprises stationary element 13 adapted to mate with removable element 14 (see FIGURE 7) which extends to an external firing circuit, and conductive sleeve 7 is the ground contact.

In FIGURE 3, the elements are as in FIGURE 2, however conductive sleeve 7 extends below the plug to provide an annulus into which the ignition charge is loaded, and reinforcing capsule 6A is added between insulation 6 and conductive sleeve 7. In this embodiment, the bridgewires 9 pass through ignition charge then are swaged between sleeve 7 and capsule 6A.

In FIGURE 4, the assembly is as in FIGURE 2, adapted for machining from a continuous length of insulated, sleeved, rod.

In FIGURE 5, the elements are as in FIGURE 2, however conductive sleeve 7 is extended down around conductor 5A to form a spark gap and bridgewire 9 is omitted,

In FIGURE 6, the ignition assembly is shown as a non-violent actuator for low-energy connecting cord which comprises core of high velocity detonating composition encased in a sheath 21 of a ductile metal. Detonator shell 1, which houses the ignition assembly of this invention, the end of coaxial conductor cable 26, and the end of the low-energy connecting cord to be actuated are contained in adapter shell 23, which may be of metal or of hard plastic. Grommet 22, which, in some cases, may be countering provided on the lowenergy connecting cord, assists in maintaining the cord axially aligned in the shell 23. Axial alignment of the detonator shell 1 and coaxial conductor 26 is assured by insulation 6 which also prohibits electrical contact between the detonator shell and/or bared conductive elements of the coaxial conductor and shell 23. The coaxial cable comprises central conductive element 26A, insulation 26B and woven conductive element 26C encased in an insulating sheath. The detonator is afiixed to coaxial conductor 26 so that central conductive element 26A is in electrical contact with conductor 5 of the ignition assembly of this invention. Woven conductive element 26C serves as the ground contact. The detonator is shown permanently assembled by swaging to coaxial conductor 26.

In FIGURES 7 and 8, suitable removable connectors are shown in greater detail. Conductive shell of the connector is adapted to mate with the detonator shell 1 or conductive sleeve 7. A length of coaxial cable 26 such as is commercially available as Amphenol or Synkote or other etxends into the connector and is held in place by crimp 27. Woven conductive element 26C of coaxial conductor is bared of insulation and is folded up over the outer insulating sheath of the coaxial conductor 26 in a conventional manner to provide the ground contact. The central conductive element 26A of cable 26 is bared of insulation 26B and extends into conductive terminal 14 which is adapted to engage with male or female terminal of stationary element 13. Shell 25 and terminal 14 are held in spaced relationship by insulation 28.

In FIGURE 9, l designates the housing, e.g., a bolt, which mates, i.e., engages, with associated element 2, e.g., a nut or otherwise one of the frame elements, for example through screw threads. Both the housing and the associated element can be attached to frame elements which can be of any desired configuration. The bore in the fastener housing 1 contains a base charge 3 of a high-velocity detonating explosive and a priming charge 4. Above the priming charge is a composite plug comprising central conductor 5, tubular insulating cylinder 6, optional reinforcing sheath 6A, insulating washer 6B, and conductive sleeve 7 which -is in contact with tube 1 which functions as ground. Bridgewires 9 attached to the central conductor pass through ignition charge 10 and are held in position by passing between insulating cylinder 6 and conductive sleeve 7 or as shown here between reinforcing sheath 6A and conductive sleeve 7. A seal 11 of pressure-compactable or extrudable material closes the bore in the housing and the bore of the fastener housing is reduced by the closing screw 16, the end of which projects inwardly to provide a rigid shoulder between the seal and the head of the fastener.

In FIGURE 10, illustrating an assembly with a single bridgewire, the elements are as in FIGURE 9, except that the closing screw is replaced by a shoulder in the housing bore and a protective plug 15, e.g., of lead, is present between base charge 3 and removable closure plug 12 to facilitate cleaning threads before installing plug 12. In FIGURE 10 the insulating cylinder 6 and insulating washer 6B are combined into one-piece. In both embodiments, the central electrical conductor 5 comprises a stationary element 13 which is enclosed in the tubular structure and means 14 which extends through the closing means and is firmly engaged by seal 11 for connecting element 13 to an outside firing circuit. The conductor 14 may be in the form of an insulated wire as shown in FIGURE 1 or a terminal pin as shown in FIGURES 3 and 4, or other configuration as will be clear to those skilled in the art. In FIGURE 10, the threads on the fastener housing are removed at the section of the housing surrounding the base charge, i.e., the section of the housing at which it is desired to sever the fastener.

As noted above, shell 1 can, in some instances, be the conductive sleeve which serves as the ground contact provided it is swaged to the insulating plug. For example, if shell 1 in the devices of FIGURES 2, 4, and 5 is of metal, the sleeve designated 7 serves as a reinforcing sheath 6A, which may be of metal or rigid plastic. As actually shown in these figures, shell 1 may be composed of metal or plastic because conductive sleeve 7 functions as the ground contact. If the shell is of plastic an external collar is swaged on as a sealing and retaining means.

In operation, when the ignition asembly of this invention is to be actuated in order to initiate an explosive train, electric current is applied to the central conductor 5 and the electrically conductive sleeve acts as the ground contact. The passage of current through the high resistance bridgewire 9, or between conductor 5A and conductive sleeve 7 in the case of the spark gap initiator, produces heat which actuates the ignition comosition 10 and thus starts the explosive train resulting in detonation of base charge 3. In embodiments such as in FIGURES 9 and 10 wherein the ignition assembly and explosive train are contained in the axial bore of a fastening element and a sealing mechanism is provided above the ignition plug, forces exerted by the detonation of the base charge are sufficient to cause the housing to be severed at the predetermined section surrounding the base charge due to expanding and stretching of the wall of the housing at this section under the influence of the detonation wave shock fronts, piling effects, and gas pressures produced upon actuation of the charge. When the wall of the housing separates. the frame elements connected by the fastener are freed and, in response to external forces acting thereon, e.g., gravity, mechanical forces, magnetic influences, the actual forces of the detonation, etc., become spatially separated. The forces of the detonation force the composite plug assembly upward so that a combination of mechanical pressure exerted by upward movement of the plug and detonation pressures acting upon the pressure-compactable mass seat the seal 11 against the rigid soulder in a fluidtight seal. Accordingly, the components in the frame elements above the head of the fastener are not exposed to detonation gases nor vented to the outside atmosphere. The central electrical conductor remains in place and is not ejected by the detonation.

The composite ignition plug assembly of this invention utilizes a central conductor which may be of any of the commonly used electrically conductive materials (metals) such as copper, silver, gold, aluminum, anodized aluminum, zinc, steel, and alloys of the above.

The electrical conductor is surrounded by a tubular mass of a relatively rigid, firm material which is electrically nonconductive. Suitable insulating materials include plastics or resins such as Bakelite, acetal resins, polyvinyl formal resins, polytetrafluoroethylene, nylon, polyvinyl fluoride, polyethylene, particularly high density, irradiated polyethylene, polyvinyl chloride, polyester resins formed by the reaction of polybasic alcohols with polyfunctional aliphatic or aromatic carboxyl-ic acids, etc. In applications where the ignition plug assembly is required to be subjected to high loading pressure without short circuiting, for example when the plug assembly is used in a self-sealing explosively releasable fastener, the tubular mass of insulating material can be backed by an insulating washer of a rigid material, such as nylon, as shown in FIGURE 9. The electrical conductor may be unitary or may be formed in two or more parts, one part being sta-tionarily embedded in the tubular mass and the other mating with the stationary part at the time of use. The two part design is particularly suitable, since fasteners of this type can be easily handled and stored without concern regarding the lengths of conductive wire extending from the fastener. The use of the two part design further decreases the hazards of premature actuation of the initiators by radio-frequency energy during storage since it eliminates long lengths of lead wires which can serve as antennae which will pick up radiofrenquency energy and induce a firing current in the initiator circuit. The firing characteristics of the ignition assembly are regulated by:

(a) The number and size of the bridgewires (when present), the minimum firing energy of the assembly increasing with an increase in the number and size of the bridgewires.

. (b) The length and diameter of the exposed portion of the central conductor, i.e., the minimum firing energy decreases with increase in length of the exposed portion and increases with increase in its diameter.

The chemical composition of the ignition material. This material can be any of those conventionally used, but particularly in the case of ignition assemblies to be used in initiators having miniature dimensions and explosive loadings, chemical compounds such as double or multiple salts are preferred since these compositions have good uniformity on a microscopic scale and are not subject to segregation. For a 1 ampere, 1 watt, 5 minute no-fire device, a magnesium/tellurium ignition composition, which can contain tellurium dioxide in particularly preferred compositions, is utilized.

(d) The physical characteristics of the ignition composition, particularly the degree of fineness and degree of compaction of the composition.

For example, a miniature electric initiator having an ignition assembly comprising a dual bridgewire of 0.001 inch diameter 80/20 platinum/iridium (resistance embedded in an ignition composition which is a chemical multiple salt of lead propionate, basic lead picrate and lead azde has a functioning time of 100 a see. when initiated by a 250 picofarad capacitor charged by 14,000 volts, i.e., when initiated by 0.0245 joule. Shorter functioning times may be obtained using the same ignition composition in conjunction with a single bridgewire of 1 mil diameter platinum-iridium having a resistance of 1.4:039 and a firing energy of 0.0125 joule, i.e., an input of 10,000 volts across a 250* picofarad capacitor. The 100 sec. timing can also be obtained using the same ignition composition and a single 0.6 mil Nichrome V bridgewire having a resistance of 17.09 at an energy input of 0.006 joule. When the latter assembly is actuated by a firing energy of 0.003 joule, the firing time is about 400 se c. Functioning times of less than 100 sec. at 0.003 joule can be achieved by using a single Nichrome bridgewire with coarse lead styphnate as the ignition composition. The all-fire direct current for this latter unit is about 0.15 ampere and it satisfies the military request for a unit responsive to a firing energy of 30,000 ergs. A 1 amp. or 1 Watt 5 minutes no-fire ignition assembly can be prepared using a 2.25 mil Nichrome bridgewire having a resistance of 0.8 0.2z in conjunction with an ignition composition comprising the magnesium/ tellurium/tellurium dioxide ignition composition. The ignition assembly is fired in an average time of 5 ,usec. upon the application of a direct current of 5 amperes either before or after being subjected to no-fire testing. The firing characteristics of each of the above assemblies can be widely varied by further changes of the parameters discussed previously.

Spark gap ignition assemblies with a variety of firing characteristics canbe prepared using the swaged coaxial ignition assembly of this invention without the bridgewires. In such assembly, any desired degree of consolidation of the ignition composition into the annular spark gap surrounding the central conductor can be achieved. Accordingly, firing characteristics not ordinarily experienced with conventional ignition materials are observed although the assembly can be used for spark gap initiation of compositions such as graphited lead azide which are normally classified as candidates for such initiation. Contemporary spark gap devices have utilized conductive mixes, i.e., lead azide containing a conductive element such as graphite. Such conductive mixes often give rise to wild variations in initiation energy particularly when used in small assemblies owing to inherent difficulties in obtaining uniform dispersion of the constituents of the mixture. Anomalous behavior is observed, for example, in the case of lead axide which is desensitized by added graphite up to a critical percentage at which abrupt increase in sensitivity occurs. Compaction and confinement of the ignition composition into the annular spark gap of the novel swaged, coaxial ignition assembly of this invention makes possible the use of a pure chemical such as lead azide or a multiple salt such as the double salt of cesium nitrate and cesium decahydrodecaborate, or the multiple salts such as basic lead acetate, lead perchlorate, and lead hydroxide (Remington No. 21 salt). For example, an ignition assembly having a 10 mil annular spark gap filled with the double salt of cesium nitrate and cesium decahydrodecaborate can be initiated with good uniformity by a firing energy of 0.006 joule. Representative of other compositions which can be used in the annular spark gap are chemically pure lead azide, commercially available as RD 1333 lead azide; the multiple salt of basic lead picrate, lead propionate and lead azide, the lead salt of dinitro-o-cresol, or any of the multipl'icity of multiple salts commercially available as Remington lead salts. As will be apparent to those skilled in the art, the swaging technique can be used to prepare flush type spark gap devices wherein the central electrode does not actually protrude into an explosive filled cavity but instead has the ignition charge positioned against the face of the flush spark gap element.

The conductive sleevewhich acts as the ground contact in the electric firing circuit can be of any electrically conductive material possessing sufficient ductility and malleability to enable the requisite swaging of the sheath onto the insulated central conductor which may be given rigidity by the use of a supplementary sheath. Particularly suitable materials for use as the sheath include copper, brass, aluminum, zinc, gold, steel, silver, and alloys of the above. The sleeve obviously should not be reactive with the ignition composition with which it is, of necessity, in intimate contact to result in its deterioration or degradation in storage or under conditions of use.

The swaged construction of our ignition assembly is particularly well suited to the manufacture of miniature bridgewire units of high reliability since it permits assembly of a multiple bridgewire type assembly with the use of at most one solder or weld point for bridgewire connection. This connection may be made by soldering, laser or resistance welding, or by a swaged or staked sleeve which mates with the extension A of the conductor. The other ends of the bridgewire are firmly imbedded in the conductive sleeve and insulating plug by a homogeneous bond. Accordingly, the assembled ignition assembly is resistant to jolts, vibration, acceleration, and water pressure and is stable in temperature cycling. Advantageously, although the swaged, coaxial plug is quite rigid in comparison to conventional rubber plugs, it retains sufiicient flexibility to allow it to be pressed into an aperture in any device in which it may be employed, this feature is lacking in other ignition assemblies which have been proposed for miniature assemblies, e.g., those incorporating glass-metal passthroughs. The use of an extended sleeve as shown in. FIGURES l, 3, 5, 9 and 10 is particularly desirable since this design allows separate loading of the ignition composition by a machine such as a rivet loader. Loose ignition materials can be pressed into the assemblies without the use of binders and, accordingly, an additional degree of control over the firing characteristics can be exercised. Since the extended conductive sleeve comprises essentially a hollow bridgepost containing all of the ignition material, there is no opportunity for accidental initiation of the assembly by double lead wire (conductor) to shell static discharge in the conventional sense.

In military applications, the use of the swaged, coaxial ignition assembly of this invention in conjunction with shielded coaxial conductors provides protection against stray currents and radio-frequency energy and can be further improved by substitution of ferrite or other semiconductor elements for all or a portion of the insulation.

The ignition assembly of this invention can be used for initiating an explosive train such as in a detonator and is particularly suitable for use with explosive trains comprising a base load of about 20 milligrams of high velocity detonating explosive and a priming charge of about milligrams of RD 1333 lead azide. The shell for such a detonator is about 0.09 inch in diameter and 0.375 inch long and can be of metal or of a plastic such as an acetal resin. As stated below, the metal shell may serve as the conductive metal sleeve for the ignition assembly. The ignition assembly also is suitable for initiating an explosive train to perforate the closure of a gas cylinder, to shatter latch pins, initiate explosive cords and the like. Because of their small size, and the correspondingly short stand-off distances that positively prevent propagation to a secondary explosive train, detonators prepared by using the ignition assembly of this invention can be used in simplified safe-arm mechanism where arming can be accomplished by the straight line action of a miniature solenoid switch of the type used in computers and business machines or where the armature of such a miniature solenoid may serve as a propagation barrier. The flush arc gap assemblies greatly facilitate the production of ionization probes for velocity of detonation measurement. Owing to the rigidity of the ignition assembly it is particularly well adapted for use in conjunction with a pressure-compactable material in a novel self-sealing, explosively-releasable fastener as shown in FIGURES 9 and 10.

When the ignition assembly is used in the novel, explosively releasable fastener, the shell 1, associated elements 2, and rigid closure elements of the fastener can be of any metal, e.g., iron, steel, other ferrous alloys, copper, bronze, aluminum, etc., or of a polymeric material such as polytetrafluoroethylene or an acetal resin (Delrin), having the structural strength and rigidity to support the load of structural elements which the fastener or fasteners connects. When a polymeric housing is used in conjunction with the composite plug as described above, a contact plate or zone of an electrically conductive material (a metal) can be provided in the section of the housing communicating with the conductive sleeve 7 to act as an electrical ground contact for the firing circuit. The thickness and strength requirements of the fastener shell naturally will depend to a large degree upon the nature of the elements to be fastened, i.e., the shell walls should have the tensile strength, impact strength and shear strength necessary to withstand vectoral forces exterted by the objects connected. The design is especially advantageous since it allows at least of the holding power of an unaltered fastener to be retained. The length of the housing will depend upon the spacing desired between structural elements. The bore provided within the shank of the fastener is made at minimum expense of the fastener strength and is provided, at the section where severing of the fastener is desired, with sufficient quantity of high velocity detonating explosive to produce a dynamic pressure pulse of sufficient force to sever, i.e., overcome the strength of, the adjacent fastener walls, yet insufiicient to cause brisance injurious to adjacent, connected structures. The term dynamic pressure pulse is used to describe that pressure of stress developed by the detonation of the charge. This pressure is a composite of factors such as the shock wave pressure, expanding gas pressure, effects, pressure reflections and the like. Its direct measurement cannot accurately be made due to its transient nature and its exceedingly high magnitudes. If desired, the threads of the housing can be removed or made thinner as shown in FIGURE 10, e.g., by machining or any other suitable means, or the housing may be notched at the section at which separation is to be effected.

In order to insure adequate severing action, the explosive employed as the base or breaking charge can be a deflagrating explosive but in order to work the greatest economy in size while still retaining maximum fastening strength must be one which detonates at a high velocity, namely a velocity greater than 1000 meters per second. While the primary detonating explosives such as lead azide may be employed, more powerful explosives, for example, pentaerythritol tetranitrate (PETN), cyclotrimethylenetrinitrarnine (RDX), cyclotetramethylenetetranitramine (HMX), nitromannite, tetryl, or TNT, exposives commonly designated as secondary explosives, are used in the preferred embodiments. Priming charges suitable for bringing about the high velocity detonation of these secondary explosives include lead azide, diazonitrophenol and mercury fulminate. A quantity as loW as 40 milligrams of secondary high explosive, used in a fastener of mild steel having a wall thickness of inch, is sufllcient to sever the fastener at the section adjacent the charge yet does not cause undesirable violence or brisance. However quantities of up to 60 milligrams of high explosive can be used, when centrally disposed in the same fastener, without loss of sealing characteristics. As mentioned above the loading in the bore will depend upon the tructural (strength) characteristics of materials used in making the fastener (materials of construction), the free space about the shank of the fastener, external leverage on the fastener, and the type of explosive employed.

The pressure compactable material disposed between the rigid composite plug and the rigid shoulder at the opening of the bore and the rigid plug in the explosively releasable fastener of this invention can be of a soft, ductile metal or plastic, or of a mass of metal powder, which undergoes plastic deformation under pressure to form a more compact mass. Suitable materials for use as this seal include lead, lead-tin alloys (which may also contain bismuth), copper-base alloys, zinc and zinc alloys. Where a harder material is needed, e.g., to withstand pressure exerted by higher explosive loadings or to aid in electric conductor retention, powered metals, for example, powders of aluminum and iron, which may be blended with one of the softer powders, can be provided. These powders may be inserted loosely and lightly compacted into place or may be held by a suitable binder such as a soft metal, an epoxy resin, etc. The seal provided when this material is compacted, i.e., seated against the rigid closure, is resistant to attenuated detonation pressures of at least 100,000 p.s.i.

Explosive charges utilized in connection with fastening structures are presently known for effecting the quick release of one element from another, e.g., in the aircraft industry for the quick release of closures, canopie and the like and for underwater use where instantaneous release of assembled parts, for example of parts of a sonar system, i required. However, the explosive-release fasteners currently available are characterized by sealed, closed breech systems which prevent the escape of explosive gases and trash at the breech; by sealed exposed systems having large over-all dimensions relative to the size of the actual fastening members where the large size is required to secure sealing after the fastener is broken; or by exposed systems which scatter shrapnel, ignition lead wires, and explosive gases into the area surrounding the entrance holes for the electrical lead wires (conductors) and which are not sealed in that location after the detonation occurs. The sealed systems have been necessary particularly in instances where it is required the the separated objects be recovered intact and neither their contents or, in other cases, the interior of the vessel from which they are separated be vented to the outside atmosphere and where no shrapnel, gases, or electric lead wires can be permitted to be ejected from the apertures for the electric lead wires. In many cases a need for sealing exists, there may be insufiicient space available for a closed breech system or for a conventional sealed exposed type fastener. Fasteners prepared in accordance with this invention can be released at a predetermined location by exposive means without destroying or undermining the strength of adjacent structures and without producing shrapnel or in any way leaking in the presence of pressure from the severed end before or after actuation. This fastener does not require a separate closedbreech system or any other enlarged members which would materially add to the cost, size, and weight of the fastener. The fastener of this invention is relatively small, e.g., essentially no larger than the fastening member, and does not require a long length to accomplish the seal, controllable, and any number of the fasteners can be separated in an essentially simultaneous action. Further, the fastener acts with a minimum of violence and produces no shattering effects upon the main assembly.

The following examples are to be considered as illustrative only and not limiting the invention in any way.

EXAMPLE 1 The ignition assemblies for 55 fasteners essentially as shown in FIGURE were prepared. In each assembly, the central conductor was of brass and was & inch long and inch in diameter. The conductor was contoured by making a inch wide neck, 0.008 inch deep, at approximately the mid-section of the conductor. The extension of the conductor 5A was formed integral with the body and was 4 inch in diameter and inch long. A hole inch deep and about 4 inch in diameter was drilled in the extremity of the conductor opposite the extension, this hole serving for solder or crimping connection of an electric terminal. A A; inch length of Thermo fit tubing (an irradiated polyethylene tubing commercial- 1y available from Rayclad Tubes Inc.) was slipped over the central conductor and the assembly was heated to 280 F. and instantaneously cooled to room temperature to provide a tight fit between the conductor and the tubing, the tubing shrinking into the necked portion of the conductor. A nylon washer inch long, 0.04 inch inner diameter and 0.080 inch outer diameter) was positioned above the conductor wit-h the axis of the washer coincident with the axis of the conductor. A copper capsule, V inch long, 0.105 inch in outer diameter, and having an aperture about 0.055 inch in diameter in the center of its base was placed about the assembled unit and swaged to an outer diameter of 0.094 inch. A single /20 platinum/iridium bridgewire having a 1 mil diameter (resistance 1.5 ohms) was soldered to the extended post of the central conductor and pulled back along side the copper capsule by slipping the assembly through a A inch length of copper tubing (0.0075 inch wall thickness) having an outer diameter of 0.115 inch. The unit then was swaged thereby reducing the outer diameter of the tube to 0.104 inch. Photomicrographs of the swaged assembly showed that the bridgewire was held in place between the plug and external copper sleeve by a homogeneous bond. The plug to sleeve joint could withstand a mechanical pull of 10 lbs. The cavity about the bridgewire was filled with loose multiple salt of basic lead picrate, lead propionate, and lead azide, commercially available as Remington No. 8 salt.

A number of the ignition assemblies were tested for firing characteristics. The. 6 inch length of Tensolon (Teflon insulated wire) conductor was connected to a direct current source and the metal sheath was connected to ground. A direct current was passed through the central conductor to the grounded metal shell with the bridgewire in place. The units withstood a no-fire current of milliamperes for five minutes and the minimum current required to fire the ignition assembly was 0.35 ampere.

The units when assembled in a x 18 NC bolt inches in length, as shown in FIGURE 10, withstood a water pressure test at 50 p.s.i. both before and after the bolts were severed by the detonation of the explosive charge.

EXAMPLE 2 Forty detonators were prepared as shown in FIGURE 3. Each detonator was made with brass terminal pins, commercial bronze sleeves and shells and irradiated polyethylene insulation and was /2 inch long and approximately 0.090 inch in diameter. The bridge plug terminal end protruded approximately /s inch from the detonator shell which was inch in length. Dual bridge wires of 80/ 20 platinum/ iridium alloy having a resistance of 0.35 i 0.1 ohm were soldered to the center post of the central conductor. Within the detonator shell was a base charge of 20 milligrams of HMX, 15 milligrams of lead azide as the priming charge, and 3 milligrams of Remington No. 8 lead salt, comprising a mixture of basic lead picrate, lead propionate, and lead azide, as the ignition composition. The coaxial, swaged plug was prepared essentionally as discussed in Example 1 except that the nylon washer Was omitted.

When the detonators were actuated by connecting the central conductor and the detonator shell in the firing circuit and applying electrostatic discharge from a 1.25 microfarad capacitor, the no-fire energy was determined to be 20,000 ergs and the all-fire energy was 50,000 ergs. When initiated by a direct current, the minimum firing current was 0.6 ampere. When the detonator was initiated by a direct current of 10 amperes, the detonators functioned in less than 150 microseconds.

As will be evident to those skilled in the art, various modifications can be made in the light of the foregoing disclosure without departing from the spirit and scope of the invention. For example, a ferrite composition can be incorporated in the swaged ignition assembly to enhance the ability of the assembly to attenuate radiofrequency energy.

What is claimed is:

1. An explosive initiator comprising a cylindrical metal shell open at one end and integrally closed at the other end, at least one explosive charge within the shell, and an ignition assembly tightly, peripherally engaged by and closing the shell at its open end, said ignition assembly being in propagating relationship to said explosive charge and comprising (a) a substantially cylindrical insulating plug,

(b) a single electrical conductor extending substantially axially through the plug and having an exposed portion extending substantially axially from the end of said plug closest to the closed end of the shell,

(c) a sleeve of electrically conductive material in intirnate peripheral engagement with the insulating p (d) an electrically conductive element of high resistance atfixed to the end of the exposed portion of the conductor and having a section of its length engaged between the conductive sleeve and the shell and homogeneously bonded directly to said sleeve, and

(e) a heat-sensitive ignition composition in intimate contact with the element of high resistance.

2. 'An explosive initiator of Clairnl wherein the metal shell has an outside diameter of up to about 0.1 inch.

3. An explosive initiator comprising a cylindrical metal shell open at one end and integrally closed at the other end, at least one explosive charge within the shell, and an ignition assembly tightly, peripherally engaged by and closing the shell at its open end, said ignition assembly being in propagating relationship to said explosive charge and comprising (a) a substantially cylindrical insulating plug composed of a central cylinder of flexible insulating material and a rigid reinforcing sheath in snug peripheral engagement with said cylinder,

(b) a single electrical conductor extending substantially axially through the plug and having an exposed portion extending substantially axially from the end of said plug closest to the closed end of the shell,

(c) an electrically conductive element of high resistance affixed to the end of the exposed portion of the conductor and having a section of its length engaged between the reinforcing sheath and the shell, and homogeneously bonded directly to said shell, and

(d) a heat-sensitive ignition composition in intimate contact with the element of high resistance.

4. An initiator of claim 3 wherein the rigid reinforcing sheath is of metal, and the outer diameter of the shell is up to about 0.1 inch.

5. An explosive device comprising a cylindrical shell open at one end and closed at the other end, at least one explosive charge within the shell, and an ignition assembly closing the open end of the shell, said ignition assembly being in propagating relationship to said explosive charge and comprising 1) a substantially cylindrical insulating plug composed of a central cylinder of flexible insulating material and a rigid reinforcing sheath in snug peripheral engagement with said cylinder,

(2) a single electrical conductor extending substantially axially through the plug and having an exposed portion extending substantially axially from the end of said plug closest to the closed end of the shell,

(3) a sleeve of electrically conductive material in intimate peripheral engagement with the rigid reinforcing sheath, the outside surface of the sleeve being peripherally engaged by said shell,

(4) an electrically conductive element of high resistance afiixed to the end of the exposed portion of the conductor and having a section of its length engaged between the conductive sleeve and said sheath, and homogeneously bonded directly to said sleeve, and

(5) a heat-sensitive ignition composition in intimate contact with the element of high resistance.

6. An explosive device of claim 5 wherein part of the conductive sleeve surrounds the exposed portion of the electrical conductor thereby forming an annulus for the ignition composition.

7. An explosive device of claim 6 wherein said reinforcing sheath is of metal.

8. An explosive device of claim 7 wherein the shell has an outside diameter of up to about 0.1 inch.

9. An explosive-release fastener comprising an elongated shank adapted to engage with associated elements, said shank having an axial bore of about A inch to about /3 inch in diameter which is closed at one end and contains in sequence from said end, at least one explosive charge, an ignition assembly in propagating relationship to said explosive charge, and means closing the other end of the bore, said ignition assembly comprising (1) a substantially cylindrical insulating plug composed of a central cylinder of flexible insulating material and a rigid reinforcing sheath in snug peripheral engagement with said cylinder,

(2) a single electrical conductor extending substantially axially through the closure means and insulating plug, and having an exposed portion extending substantially axially from the end of said plug closest to the explosive charge,

(3) a sleeve of electrically conductive material in intimate peripheral engagement with the rigid reinforcing sheath and in contact with means for grounding the ignition assembly, part of said sleeve surrounding the exposed portion of the electrical conductor thereby forming an annulus,

(4) an electrically conductive element of high resistance afiixed to the end of said exposed portion and having a section of its length engaged between the conductive sleeve and reinforcing sheath, and homogeneously bonded directly to said sleeve, and

(5) a heat-sensitive ignition composition in said annulus and in intimate contact with the element of high resistance.

References Cited by the Examiner UNITED STATES PATENTS 1,605,688 11/1926 Olin et al l02--28 2,681,701 6/1954 Schlumberger 10228 X 2,687,667 8/1954 Gunther 86--1 2,721,913 10/1955 Kent 102-702 X 2,921,520 1/ 1960 Stonestrom 102-28 3,040,284 6/ 1962 Connell. 3,044,342 7/1962 Jones 86-1 3,062,146 11/1962 Williams et al. 10270.2 X 3,094,932 6/1963 Greenlees 102-28 3,107,613 10/1963 Armstrong et al. 102--28 3,196,746 7/1965 Dahl 89-1.5 X

FOREIGN PATENTS 556,655 4/1958 Canada. 1,246,888 10/1960 France.

785,745 11/ 1957 Great Britain.

BENJAMIN A. BORCHELT, Primary Examiner.

SAMUEL FEINBERG, Examiner.

G. L. PETERSON, G. H. GLANZMAN,

Assistant Examineray. 

3. AN EXPLOSIVE INITIATOR COMPRISING A CYLINDRICAL METAL SHELL OPEN AT ONE END AND INTEGRALLY CLOSED AT THE OTHER END, AT LEAST ONE EXPLOSIVE CHARGE WITHIN THE SHELL, AND AN IGNITION ASSEMBLY TIGHTLY, PERIPHERALLY ENGAGED BY AND CLOSING THE SHELL AT ITS OPEN END, SAID IGNITION ASSEMBLY BEING IN PROPAGATING RELATIONSHIP TO AND EXPLOSIVE CHARGE AND COMPRISING (A) A SUBSTANTIALLY CYLINDRICAL INSULATING PLUG COMPOSED OF A CENTRAL CYLINDER OF FLEXIBLE INSULATION MATERIAL AND A RIGID REINFORCING SHEATH IN SNUG PERIPHERAL ENGAGEMENT WITH SAID CYLINDER, (B) A SINGLE ELECTRICAL CONDUCTOR EXTENDING SUBSTANTIALLY AXIALLY THROUGH THE PLUG AND HAVING AN EXPOSED PORTION EXTENDING SUBSTANTIALLY AXIALLY FROM THE END OF SAID PLUG CLOSET TO THE CLOSED END OF THE SHELL, (C) AN ELECTRICALLY CONDUCTIVE ELEMENT OF HIGH RESISTANCE AFFIXED TO THE END OF THE EXPOSED PORTION OF THE CONDUCTOR AND HAVING A SECTION OF ITS LENGTH ENGAGED BETWEEN THE REINFORCING SHEATH AND THE SHELL, AND HOMOGENEOUSLY BONDED DIRECTLY TO SAID SHELL, AND (D) A HEAT-SENSITIVE IGNITION COMPOSITION IN INTIMATE CONTACT WITH THE ELEMENT OF HIGH RESISTANCE. 